Process for the preparation of ethylene homopolymers or copolymers

ABSTRACT

A process for the preparation of ethylene homopolymers or copolymers in a facility having a high-pressure tubular reactor and a preheater, wherein a reaction fluid introduced into the reactor at a reactor inlet is heated in the preheater and the average velocity of the reaction fluid in the preheater is lower than the average velocity of the reaction fluid in the tubular reactor and the ratio of the average velocity in the tubular reactor to the average velocity of the reaction fluid in the preheater is in the range from 1.5 to 5.

FIELD OF THE INVENTION

In general, the present disclosure relates to the field of chemistry. More specifically, the present disclosure relates to polymer chemistry. In particular, the present disclosure provides a process for the preparation of ethylene homopolymers or copolymers.

BACKGROUND OF THE INVENTION

In some instances, ethylene polymers are formed in high-pressure polymerization processes. Process steps include compression of the reaction mixture to the reactor pressure, introduction of an initiator in at least one stage, polymerization, removal of exothermic heat of reaction, and product separation. For the processes, different types of reactor design are used. In some instances, reactors to form low density ethylene-based polymers are tubular reactors.

In some instances and for preparing low density ethylene homopolymers or copolymers, the reaction components are pressurized by a combination of two compressors, a primary compressor and a secondary compressor. In some instances and at the end of the process, the equipment also includes extruders and granulators for pelletizing the obtained polymer. In some instances, the equipment also provides for feeding monomers and comonomers, free-radical initiators, chain transfer agents or other substances at one or more positions to the polymerization reaction.

In some instances, the reaction gas composition discharged by the secondary compressor and supplied to the inlet of the reactor is heated. In some instances, the heating is achieved by a heating jackets operated by steam, hot water or a combination thereof. These heating elements for heating the reaction gas composition before arriving at the reactor inlet are a preheater.

The properties and the structure of the produced ethylene homopolymers or copolymers, such as molecular weight, molecular weight distribution and the amount of short- and long-chain branching, depend on the reaction parameters. Furthermore, the feasible ethylene conversion in a reaction zone depends on the amount of polymerization heat, which can be removed from the reaction mixture in the respective reaction zone.

SUMMARY OF THE INVENTION

In a general embodiment, the present disclosure provides a process for the preparation of ethylene homopolymers or copolymers in a facility having i) a high-pressure tubular reactor used in a continuous flow mode with a reaction fluid made from or containing ethylene being introduced into the reactor at a reactor inlet and polymer leaving the reactor at a reactor outlet and ii) a preheater having an inlet and an outlet, including the steps of:

-   a) compressing the reaction fluid to an elevated pressure; -   b) heating at least a portion of the reaction fluid in the     preheater; -   c) introducing the reaction fluid heated in step b) into the reactor     at the reactor inlet; and -   d) polymerizing the reaction fluid at the elevated pressure in the     presence of free-radical polymerization initiators in the reactor,     thereby yielding a polymer selected from the group consisting of     ethylene homopolymers and ethylene copolymers,     wherein the average velocity of the reaction fluid at the outlet of     the preheater is lower than the average velocity of the reaction     fluid in the tubular reactor at the reactor inlet and the ratio of     the average velocity of the reaction fluid in the tubular reactor at     the reactor inlet to the average velocity of the reaction fluid in     the preheater at the outlet of the preheater is in the range from     1.5 to 5.

In some embodiments, in step a), a piston compressor is used for compressing of the reaction fluid.

In some embodiments, in step a), the pressure of the reaction fluid after compression ranges from 180 MPa to 320 MPa. In some embodiments, in step c), when introducing the heated reaction fluid into the reactor, the pressure of the reaction fluid ranges from 175 MPa to 315 MPa.

In some embodiments, in step c), when introducing the heated reaction fluid into the reactor, the temperature of the reaction fluid ranges from 140° C. to 170° C.

In some embodiments, the preheater includes two preheater units, wherein the first preheater unit is arranged on a first branch line and the second preheater unit is arranged on a second branch line.

In some embodiments, the process includes the following steps:

-   a′) providing a first flow of compressed reaction fluid made from or     containing a first portion of the reaction fluid and providing a     second flow of compressed reaction fluid made from or containing a     second portion of the reaction fluid by, in any order, compressing     the reaction fluid to an elevated pressure and splitting at least a     portion of a single flow of the reaction fluid into the first flow     and the second flow of compressed reaction fluid; -   b′) conducting the first flow of the first portion of the reaction     fluid through the first preheater unit and heating the first portion     of the reaction fluid in the first preheater unit, and conducting     the second flow of the second portion of the reaction fluid through     the second preheater unit and heating the second portion of the     reaction fluid in the second preheater unit; -   c′) combining the first and second portion of the reaction fluid by     recombining the first flow and the second flow of the reaction fluid     and introducing the recombined flow of the reaction fluid heated in     step b′) into the reactor at the reactor inlet; and -   d′) polymerizing the reaction fluid at the elevated pressure in the     presence of free-radical polymerization initiators in the reactor.

In some embodiments, in step a′), a single flow of the reaction fluid is compressed to an elevated pressure and thereafter at least a portion of the flow is split at a junction appliance, into the first flow made from or containing the first portion of the reaction fluid and into the second flow made from or containing the second portion of the reaction fluid.

In some embodiments, the first preheater unit has first heating elements and the second preheater unit has second heating elements that are arranged at intervals and spaced apart from each other.

In some embodiments, the first and second heating elements are heated using a heating fluid, at least some of the first heating elements are connected by heating pipelines to each other, and at least some of the second heating elements are connected by heating pipelines to each other.

In some embodiments, the first heating elements have at least two subsets of first heating elements, wherein the first heating elements within each subset are connected by heating pipelines, the subsets are not connected to each other by heating pipelines, different heating fluids are used for heating each subset of first heating elements,

the second heating elements have at least two subsets of second heating elements, wherein the second heating elements within each subset are connected by heating pipelines, the subsets are not connected to each other by heating pipelines, different heating fluids are used for heating each subset of second heating elements, and

the different heating fluids differ in a property selected from the group consisting of pressure, composition, source of the heating fluid, and temperature.

In some embodiments, the amount of heating elements arranged in or around the first and second branch line is the same. In some embodiments, the length of the first and second branch line is the same.

In some embodiments, the first and second branch lines have straight sections that are partially or fully insulated using insulation elements and the bends are not insulated with the insulation elements.

In some embodiments, the polymer obtained in step d) is a low-density polyethylene (LDPE).

In some embodiments, the reaction fluid is further made from or containing propionic aldehyde or propylene.

In some embodiments, the reaction fluid is a supercritical fluid in steps b) to d).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows schematically a set-up of a facility for carrying out a high-pressure polymerization.

FIG. 2 shows schematically a preheater unit.

FIG. 3 shows schematically an arrangement of a first and a second preheater unit in connection with a tubular reactor.

DETAILED DESCRIPTION OF THE INVENTION

In some embodiments, the present disclosure provides a process for the preparation of ethylene homopolymers or copolymers in a facility having a high-pressure tubular reactor. In some embodiments, the high-pressure polymerization for preparing the ethylene homopolymers or copolymers is carried out at pressures of from 110 MPa to 500 MPa, alternatively from 160 MPa to 350 MPa, alternatively from 200 MPa to 330 MPa. In some embodiments, the polymerization temperatures are in the range of from 100° C. to 350° C., alternatively from 180° C. to 340° C., alternatively from 200° C. to 330° C. for polymerization.

In some embodiments, the reaction fluid made from or containing ethylene is brought to the polymerization pressure by one or more compressors in a sequence of compression stages. As used herein and in some embodiments, the term “fluid” refers to a gas or a liquid, including a supercritical fluid.

A reaction mixture made from or containing polymer obtained by the polymerization leaves the reactor at the reactor outlet. In some embodiments, the reaction mixture leaves through a pressure control valve. In some embodiments, the reaction mixture is cooled by a post reactor cooler. In some embodiments, the reaction mixture leaving the reactor is separated into polymeric and gaseous components in two or more stages, where the gaseous components separated off in a first stage at an absolute pressure of from 15 MPa to 50 MPa are recycled to the one or more compressors via a high-pressure gas recycle line and the gaseous components separated off in a second stage at an absolute pressure in the range of from 0.1 MPa to 0.5 MPa are recycled to the first stage of the sequence of compression stages via a low-pressure gas recycle line, and the polymeric components obtained by the polymerization are transformed into pellets.

In some embodiments, the high-pressure polymerization is a homopolymerization of ethylene or a copolymerization of ethylene with one or more other monomers, provided that these monomers are free-radically copolymerizable with ethylene under high pressure. In some embodiments, the copolymerizable monomers are selected from the group consisting of α,β-unsaturated C₃-C₈-carboxylic acids, derivatives of α,β-unsaturated C₃-C₈-carboxylic acids, and 1-olefins. In some embodiments, the derivatives of α,β-unsaturated C₃-C₈-carboxylic acids are unsaturated C₃-C₁₅-carboxylic esters or anhydrides. In some embodiments, the copolymerizable monomers are vinyl carboxylates. In some embodiments, the vinyl carboxylate is vinyl acetate. In some embodiments, the copolymerizable monomers are selected from the group consisting of propene, 1-butene, 1-hexene, acrylic acid, n-butyl acrylate, tert-butyl acrylate, 2-ethylhexyl acrylate, vinyl acetate and vinyl propionate.

In the case of copolymerization, the proportion of comonomer or comonomers in the reaction mixture is from 1 to 50% by weight, alternatively from 3 to 40% by weight, based on the amounts of monomers, that is, the sum of ethylene and other monomers. In some embodiments, the comonomers are fed at more than one point to the reactor set-up. In some embodiments, the comonomers are fed to the suction side of the secondary compressor.

As used herein, the terms “polymers” or “polymeric materials” refer to substances which are made up of at least two monomer units. In some embodiments, the polymers or polymeric materials are low density polyethylenes having an average molecular weight M_(n) of more than 20 000 g/mole. As used herein, the term “low density polyethylene” (LDPE) includes ethylene homopolymers and ethylene copolymers. In some embodiments, the process is used to prepare oligomers, waxes and polymers having a molecular weight M_(n) of less than 20 000 g/mole.

In some embodiments, the polymerization is a radical polymerization carried out in the presence of free-radical polymerization initiators. In some embodiments, initiators for starting the polymerization in the respective reaction zones are substances that produce radical species under the conditions in the polymerization reactor. In some embodiments, the initiators are selected from the group consisting of oxygen, air, azo compounds and peroxidic polymerization initiators. In some embodiments, the polymerization is carried out by using oxygen, either fed in the form of pure O₂ or as air. In some embodiments and with oxygen as an initiator, the initiator is first mixed with the ethylene feed and then fed to the reactor. In some embodiments, a stream made from or containing monomer and oxygen is fed to the beginning of the polymerization reactor and one or more points along the reactor creating two or more reaction zones. In some embodiments, initiation uses organic peroxides or azo compounds. In some embodiments, the initiators are used as individual initiators. In some embodiments, the initiators are used as mixtures of various initiators. In some embodiments, the initiators are commercially available from Akzo Nobel under the trade names Trigonox® or Perkadox®.

In some embodiments, and in the high-pressure polymerization, the molecular weight of the polymers to be prepared is altered by the addition of modifiers which act as chain-transfer agents. In some embodiments, the modifiers are selected from the group consisting of hydrogen, aliphatic and olefinic hydrocarbons, ketones, aldehydes, and saturated aliphatic alcohols. In some embodiments, the hydrocarbons are selected from the group consisting of propane, butane, pentane, hexane, cyclohexane, propene, 1-butene, 1-pentene, and 1-hexene. In some embodiments, the ketones are selected from the group consisting of acetone, methyl ethyl ketone (2-butanone), methyl isobutyl ketone, methyl isoamyl ketone, diethyl ketone, and diamyl ketone. In some embodiments, the aldehydes are selected from the group consisting of formaldehyde, acetaldehyde, and propionaldehyde. In some embodiments, the saturated aliphatic alcohols are selected from the group consisting of methanol, ethanol, propanol, isopropanol, and butanol. In some embodiments, the saturated aliphatic aldehyde is propionaldehyde. In some embodiments, the 1-olefins are selected from the group consisting of propene, 1-butene, and 1-hexene. In some embodiments, the aliphatic hydrocarbon is propane.

In some embodiments, the tubular reactors are long, thick-walled pipes. In some embodiments, tubular reactors are from about 0.5 km to 4 km, alternatively from 1 km to 3 km, alternatively from 1.5 km to 2.5 km long. In some embodiments, the inner diameter of the pipes is in the range of from about 30 mm to 120 mm, alternatively from 60 mm to 100 mm. In some embodiments, the tubular reactors have a length-to-diameter ratio of greater than 1000, alternatively from 10000 to 40000, alternatively from 25000 to 35000. In some embodiments and when feeding part of the compressed reaction fluid as one or more side streams to the tubular reactor downstream of the inlet of the tubular reactor, the tubular reactor lacks a constant diameter over the full length while having an increasing diameter downstream of each injection point for a reaction fluid stream.

In some embodiments, the tubular reactor is composed of individual segments. In some embodiments, the segments or tubings are flanged together. In some embodiments, the tubings are straight tubes. In some embodiments, the tubings are bends. In some embodiments, the straight tubes have a length of from 15 m to 20 m. In some embodiments, the tubes are flanged to other tubes. In some embodiments, the tubes are flanged to a bend. In some embodiments, the bends are 180° bend bends, that is, bends shaped in a semicircle. In some embodiments, the bends are formed such that more than one bend flanged together form a semicircle. In some embodiments, two bends are flanged together. In some embodiments, the bends have a small radius. In some embodiments, the centerline radius of the bends is not more than tenfold of the average inside diameter of the bends, alternatively the centerline radius of the bends is not more than eightfold of the average inside diameter of the bends, alternatively the centerline radius of the bends is not more than fivefold of the average inside diameter of the bends.

In some embodiments, the flanges are arranged such that groups of flanges are aligned on top of each other. In some embodiments, a group of flanges is arranged one atop of the other and has at least two flanges, alternatively from 3 to 100 flanges, alternatively from 5 to 60 flanges.

In some embodiments, the tubular reactors have at least two reaction zones, alternatively from 2 to 6 reaction zones, alternatively from 2 to 5 reaction zones. The number of reaction zones is given by the number of feeding points for the initiator. In some embodiments, a feeding point is for a solution of azo compounds or organic peroxides. Fresh initiator is added to the reactor, where the initiator decomposes into free radicals and initiates further polymerization. It is believed that the generated heat of the reaction raises the temperature of the reaction mixture because more heat is generated than is removed through the walls of the tubular reactor. The rising temperature increases the rate of decomposition of the free-radical initiators and accelerates polymerization until the free-radical initiator is consumed. Thereafter, no further heat is generated. The temperature decreases because the temperature of the reactor walls is lower than that of the reaction mixture. The part of the tubular reactor downstream of an initiator feeding point wherein the temperature rises is the reaction zone. The part wherein the temperature decreases is a cooling zone. The amount and nature of added free-radical initiators determines how much the temperature rises and accordingly allows for adjusting that value. In some embodiments, the temperature rise is set to be in the range of from 70° C. to 170° C. in the first reaction zone and 50° C. to 130° C. for the subsequent reaction zones depending on the product specifications and the reactor configuration. In some embodiments, the tubular reactor is equipped with cooling jackets for removing the heat of the reaction. In some embodiments, the reaction zones of the tubular reactor are cooled by cooling jackets.

In some embodiments, the cooling jackets are arranged on straight tubes. In some embodiments, each of the straight tubes is equipped with a cooling jacket. In some embodiments, water is used as a cooling medium. In some embodiments, some adjacent straight tubes form a group, which is supplied with the cooling medium. In some embodiments, a group includes two, three, four, five or six straight tubes. The cooling medium is then conveyed to the cooling jacket of one of the straight tubes of the group and thereafter passed through the cooling jackets of the other members of the group of straight tubes before being taken away from the tubular reactor.

In some embodiments, the compression of the reaction fluid to the elevated pressure, at which the polymerization is carried out, occurs by one or more compressors in a sequence of compression stages. In some embodiments, a primary compressor first compresses the reaction fluid to a pressure of from 10 MPa to 50 MPa and a secondary compressor further compresses the reaction fluid to the polymerization pressure of from 110 MPa to 500 MPa. In some embodiments, the secondary compressor is designated as a hyper compressor. In some embodiments, the primary compressor and the secondary compressor are multistage compressors. In some embodiments, one or more stages of one or both of these compressors are separated into separated compressors. In some embodiments, a series of one primary compressor and one secondary compressor is used for compressing the reaction fluid to the polymerization pressure. In some embodiments, the whole primary compressor is designated as primary compressor. In some embodiments, the one or more first stages of the primary compressor, which compress the recycle gas from the low-pressure product separator to the pressure of the fresh ethylene feed, are designated the booster compressor and the one or more subsequent stages are designated as the primary compressor, although the booster compressor and the subsequent stages are part of one apparatus. Due to the technical concept of reciprocating compressors, common secondary compressors do not provide a constant pressure and the pressure level is oscillating as a result of the pulsating flow.

In some embodiments, the facility has a preheater upstream of the tubular reactor for heating the reaction fluid to a temperature capable of initiating the polymerization. In some embodiments, the preheater has an inlet into which the reaction fluid is introduced for being heated and an outlet through which the heated reaction fluid leaves the preheater. In some embodiments, the preheater is a long, thick-walled pipe having an inner diameter from 30 mm to 140 mm, alternatively from 60 mm to 120 mm. In some embodiments, the preheater has individual segments. In some embodiments, the segments or tubings are flanged together. In some embodiments, the preheating tubings are straight tubes. In some embodiments, the preheating tubings are bends. In some embodiments, the straight tubes have a length of from 15 m to 20 m. In some embodiments, the tubes are flanged to other tubes. In some embodiments, the tubes are flanged to a bend. In some embodiments, bends are 180° bend bends, that is, bends shaped in a semicircle. In some embodiments, the bends are formed such that more than one bend is flanged together to form a semicircle. In some embodiments, two bends are flanged together. In some embodiments, the bends have a small radius. In some embodiments, the centerline radius of the bends is not more than tenfold of the average inside diameter of the bends, alternatively the centerline radius of the bends is not more than eightfold of the average inside diameter of the bends, alternatively the centerline radius of the bends is not more than fivefold of the average inside diameter of the bends. In some embodiments, the flanges are arranged such that groups of flanges are aligned on top of each other. In some embodiments, a group of flanges is arranged one atop of the other and has at least two flanges, alternatively from 3 to 50 flanges, alternatively from 5 to 30 flanges.

In some embodiments, the reaction fluid provided by the secondary compressor is fed via the preheater to the inlet of the tubular reactor. In some embodiments, part of the reaction fluid compressed by the secondary compressor is fed via the preheater to the inlet of the tubular reactor and the remainder of the reaction fluid compressed by the secondary compressor is fed as one or more side streams to the tubular reactor downstream of the inlet of the tubular reactor. In some embodiments, from 30 to 90% by weight, alternatively from 40 to 70% by weight, of the reaction fluid provided by the secondary compressor is fed to the inlet of the tubular reactor. In some embodiments, from 10 to 70% by weight, alternatively from 30 to 60% by weight, of the reaction fluid provided by the secondary compressor is fed as one or more side streams to the tubular reactor downstream of the inlet of the tubular reactor.

The facility for carrying out the polymerization further includes two or more gas recycle lines for recycling unreacted monomers into the polymerization process. The reaction mixture obtained in the polymerization reactor is transferred to a first separation vessel and separated into a gaseous fraction and a liquid fraction at an absolute pressure of from 15 MPa to 50 MPa. In some embodiments, the first separation vessel is called a high-pressure product separator. The gaseous fraction withdrawn from the first separation vessel is fed via a high-pressure gas recycle line to the suction side of the secondary compressor. In some embodiments and in the high-pressure gas recycle line, the gas is purified by several purification steps for removing components such as entrained polymer or oligomers. The liquid fraction withdrawn from the first separation vessel is transferred to a second separation vessel and further separated, at reduced pressure, in polymeric and gaseous components. In some embodiments, the liquid fraction is made from or containing dissolved monomers such as ethylene and comonomers in an amount of 20 to 40% of weight. In some embodiments, the second separation vessel is called a low-pressure product separator. In some embodiments, the reduced pressure is an absolute pressure in the range of from 0.1 MPa to 0.5 MPa. The gaseous fraction withdrawn from the second separation vessel is fed via a low-pressure gas recycle line to the primary compressor, alternatively to the foremost of the stages. In some Membodiments, the low-pressure gas recycle line includes several purification steps for purifying the gas. In some embodiments, the facility includes additional separation steps for separating additional gaseous fractions from the reaction mixtures and additional gas recycle lines for feeding such additional gaseous fractions including unreacted monomers to one of the compressors operating at an intermediate pressure. In some embodiments, the additional gaseous fractions are fed in-between the first separation step and the second separation step.

In some embodiments, the recycled gas coming from the low-pressure gas recycle line is compressed by the first stages of the primary compressor to the pressure of the fresh feed of ethylenically unsaturated monomers and thereafter combined with the fresh gas feed. The combined gases are further compressed in the primary compressor to the pressure of from 10 MPa to 50 MPa. In some embodiments, the ethylenically unsaturated monomer is ethylene. In some embodiments, the primary compressor includes five or six compression stages, two or three before adding the fresh gas and two or three after adding the fresh gas. In some embodiments, the secondary compressor has two stages; a first stage, which compresses the gas from about 30 MPa to about 120 MPa, and a second stage, which further compresses the gas from about 120 MPa to the final polymerization pressure.

In some embodiments, the pressure within the polymerization reactor is controlled by a pressure control valve, which is arranged at the outlet of the polymerization reactor and through which the reaction mixture leaves the reactor. In some embodiments, the pressure control valve is a valve arrangement for reducing the pressure of the reaction mixture leaving the reactor to the pressure within the first separation vessel.

In some embodiments, the facility includes a post reactor cooler downstream of the polymerization reactor for cooling the reaction mixture. In some embodiments, the post reactor cooler is arranged upstream of the pressure control valve. In some embodiments, the post reactor cooler is arranged downstream of the pressure control valve. In some embodiments, the post reactor cooler is arranged downstream of the pressure control valve. In some embodiments, the post reactor cooler is a long, thick-walled pipe having an inner diameter from 30 mm to 120 mm, alternatively from 60 mm to 100 mm. In some embodiments, the post reactor cooler has individual segments. In some embodiments, these segments or cooling tubings are flanged together. In some embodiments, the cooling tubings are straight tubes. In some embodiments, the cooling tubings are bends. In some embodiments, the straight tubes have a length of from 15 m to 20 m. In some embodiments, the tubes are flanged to other tubes. In some embodiments, the tubes are flanged to a bend. In some embodiments, bends are 180° bends, that is, bends shaped in a semicircle. In some embodiments, the bends are formed such that more than one bend flanged together to form a semicircle. In some embodiments, two bends are flanged together. In some embodiments, the bends have a small radius. In some embodiments, the centerline radius of the bends is not more than tenfold of the average inside diameter of the bends, alternatively the centerline radius of the bends is not more than eightfold of the average inside diameter of the bends, alternatively the centerline radius of the bends is not more than fivefold of the average inside diameter of the bends. In some embodiments, the flanges are arranged such that groups of flanges are aligned on top of each other. In some embodiments, a group of flanges is arranged one atop of the other and has at least two flanges, alternatively from 3 to 80 flanges, alternatively from 5 to 60 flanges.

In some embodiments, the tubular reactors are located in a chamber having a surrounding protective wall.

The polymeric components obtained by the polymerization are finally transformed into pellets. In some embodiments, the pellets are formed using apparatuses like extruders or granulators.

FIG. 1 shows schematically a set-up of a facility for high-pressure ethylene polymerization having a tubular reactor which is continuously operated.

In some embodiments, the fresh ethylene, under a pressure of 1.7 MPa, is firstly compressed to a pressure of about 30 MPa by a primary compressor (1 a*) and then compressed to the reaction pressure of about 300 MPa using a secondary compressor (1 b*). In some embodiments, chain transfer agents (CTA) are added to the primary compressor (1 a*) together with the fresh ethylene. In some embodiments, comonomer is added upstream of the secondary compressor (1 b*). The reaction mixture leaving the secondary compressor (1 b*) is fed to a preheater (2*), where the reaction mixture is preheated to the reaction start temperature of from about 120° C. to 220° C., and then conveyed to the inlet (3*) of the tubular reactor (4*).

In some embodiments, the tubular reactor (4*) is a long, thick-walled pipe with cooling jackets to remove the liberated heat of reaction from the reaction mixture by a coolant circuit (not shown).

The tubular reactor (4*) has four spatially separated initiator feeding points (5 a*), (5 b*), (5 c*) and (5 d*) for feeding initiators or initiator mixtures PX1, PX2, PX3 and PX4 to the reactor and four reaction zones. By feeding free-radical initiators, which decompose at the temperature of the reaction mixture, to the tubular reactor, the polymerization reaction starts.

The reaction mixture leaves the tubular reactor (4*) through a pressure control valve (6*) and passes a post reactor cooler (7*). Thereafter, the resulting polymer is separated off from unreacted ethylene and other low molecular weight compounds by a first separation vessel (8*) and a second separation vessel (9*), discharged, and pelletized via an extruder and granulator (10*). In some embodiments, the low molecular weight compounds include monomers, oligomers, polymers, additives, or solvent.

The ethylene and comonomers which have been separated off in the first separation vessel (8*) are fed back to the inlet end of the tubular reactor (4*) in the high-pressure circuit (11*) at 30 MPa. In the high-pressure circuit (11*), the gaseous material separated from the reaction mixture is first freed from other constituents in at least one purification stage and then added to the monomer stream between primary compressor (1 a*) and secondary compressor (1 b*). FIG. 1 shows a purification stage consisting of a heat exchanger (12*) and a separator (13*). In some embodiments, a plurality of purification stages is used. In some embodiments, the high-pressure circuit (11*) separates waxes.

The ethylene which has been separated off in the second separation vessel (9*), along with the major part of the low molecular weight products of the polymerization (oligomers) and the solvent, is worked up in the low-pressure circuit (14*) at an absolute pressure of from about 0.1 MPa to 0.5 MPa in a plurality of separators with a heat exchanger being installed between each of the separators. FIG. 1 shows two purification stages having heat exchangers (15*) and (17*) and separators (16*) and (18*). In some embodiments, one purification stage is used. In some embodiments, more than two purification stages are used. In some embodiments, the low-pressure circuit (14*) separates oil and waxes.

In some embodiments, the the process for the preparation of ethylene homopolymers or copolymers includes the following steps:

-   a) compressing the reaction fluid made from or containing ethylene,     alternatively an ethylene gas, to an elevated pressure; -   b) heating at least a portion of the reaction fluid in the     preheater; -   c) introducing the reaction fluid heated in step b) into the reactor     at the reactor inlet; and -   d) polymerizing the reaction fluid at the elevated pressure in the     presence of free-radical polymerization initiators in the reactor,     thereby yielding a polymer selected from the group consisting of     ethylene homopolymers and ethylene copolymers,     wherein the average velocity of the reaction fluid at the outlet of     the preheater is lower than the average velocity of the reaction     fluid in the tubular reactor at the reactor inlet and the ratio of     the average velocity of the reaction fluid in the tubular reactor at     the reactor inlet to the average velocity of the reaction fluid in     the preheater at the outlet of the preheater is in the range from     1.5 to 5, alternatively from 1.7 to 2, alternatively from 1.8 to     2.2. It is believed that operating the preheater with a lower     average velocity of the reaction fluid in the preheater than in the     tubular reactor ensures a low pressure drop over the preheater,     thereby allowing an increased conversion rate and a reduced pressure     pulsation level within the tubular reactor.

In some embodiments and as used herein, the average velocity of the reaction fluid at a specific position of the tube is calculated from the average mass flow through the cross-sectional area at the specific position of the tube in consideration of the reaction fluid density at the given pressure and temperature. The average velocity of the reaction fluid is a mean over variations in the mass flow and a mean over the velocity profile which forms across the diameter of the tube when a fluid flows through a tube. In some embodiments, the variations in the mass flow are caused by pressure fluctuations induced by the secondary compressor.

In some embodiments, in step a), a piston compressor is used for compressing of the reaction fluid. In some embodiments, the compression is affected by one or more multiple-stage, piston compressors. In some embodiments, a compressor is a two-stage piston compressor. In some embodiments, the piston compressor is a compressor that is a high-pressure reciprocating compressor. In some embodiments, the piston compressor achieves a pressure of at least 180 MPa, alternatively at least 250 MPa.

In some embodiments, in step a), the pressure of the reaction fluid after compression ranges from 180 MPa to 320 MPa. In some embodiments, in step c), when introducing the heated reaction fluid into the reactor, the pressure of the reaction fluid ranges from 175 MPa to 315 MPa. In some embodiments, the pressure in step a) is higher than the pressure in step c). In some embodiments, the pressure in step a) is not more than 10%, alternatively 5%, alternatively 2%, higher than the pressure in step c) when introducing the heated reaction fluid into the reactor. In some embodiments, the pressure in step a) is higher than the pressure in step c) by an absolute pressure difference of from 0.1 to 20 MPa, alternatively from 0.5 to 10 MPa, alternatively from 1 to 6 MPa. In some embodiments, the absolute pressure difference is less than 5 MPa.

In some embodiments, in step c), when introducing the heated reaction fluid into the reactor, the temperature of the reaction fluid ranges from 140° C. to 170° C.

In some embodiments, the preheater has two preheater units, wherein the first preheater unit is arranged on a first branch line, the second preheater unit is arranged on a second branch line, and the first and second preheater units are installed in parallel. In some embodiments, the two preheater units are pipes having an identical diameter. In some embodiments, the two preheater units are pipes having the same diameter as the tubular reactor.

In some embodiments, the preheater has two preheater units, wherein the first preheater unit is arranged on a first branch line, the second preheater unit is arranged on a second branch line, and the first and second preheater units are installed in parallel, and the process for the preparation of ethylene homopolymers or copolymers includes the following steps:

-   a′) providing a first flow of compressed reaction fluid made from or     containing a first portion of the reaction fluid and providing a     second flow of compressed reaction fluid made from or containing a     second portion of the reaction fluid by, in any order, compressing     the reaction fluid to an elevated pressure and splitting at least a     portion of a single flow of the reaction fluid into the first flow     and the second flow of compressed reaction fluid; -   b′) conducting the first flow of the first portion of the reaction     fluid through the first preheater unit and heating the first portion     of the reaction fluid in the first preheater unit, and conducting     the second flow of the second portion of the reaction fluid through     the second preheater unit and heating the second portion of the     reaction fluid in the second preheater unit; -   c′) combining the first and second portion of the reaction fluid by     recombining the first flow and the second flow of the reaction fluid     and introducing the recombined flow of the reaction fluid heated in     step b′) into the reactor at the reactor inlet; and -   d′) polymerizing the reaction fluid at the elevated pressure in the     presence of free-radical polymerization initiators in the reactor.

In some embodiments, in step a′), a single flow of the reaction fluid is compressed to an elevated pressure and thereafter at least a portion of the flow is split at a junction appliance, into the first flow made from or containing the first portion of the reaction fluid and into the second flow made from or containing the second portion of the reaction fluid. In some embodiments in step a′) a single flow of the reaction fluid made from or containing ethylene is split at a junction appliance into a first flow made from or containing the first portion of the reaction fluid and into a second flow made from or containing the second portion of the reaction fluid and thereafter the first portion of the reaction fluid and second portion of the reaction fluid are compressed using two compressors, and in step b′) the first flow is conducted through the first preheater unit and the second flow is conducted through the second preheater unit, and in step c′) the flows are recombined.

In some embodiments, the recombination is achieved in a merging appliance, wherein a single line leads to the reactor. In some embodiments, the merging appliance is a T-piece.

In some embodiments, the first preheater unit has first heating elements and the second preheater unit has second heating elements that are arranged at intervals and spaced apart from each other. In some embodiments, the heating elements are connected by heating pipelines. In some embodiments, the heating elements are not connected by heating pipelines. As used herein and in some embodiments, the heating elements that are connected by heating pipelines form a subset of connected heating elements. A heating pipeline is a channel that transports a heating fluid to heat the heating elements. The heating pipeline is not identical with the first and second branch line for the reaction fluid. The heating pipeline does not carry reaction fluid. The heating pipeline carries a heating fluid having a different composition than the reaction fluid, which is used for heating of the branch lines. In some embodiments, the heating fluid is liquid water or steam. In some embodiments, the heating fluid is steam, alternatively saturated steam. In some embodiments, the heating of the branch lines occurs by flowing through heating elements arranged around the branch line(s), alternatively around the first or second branch line

In some embodiments, the first and second heating elements are heated using a heating fluid, wherein at least some of the first heating elements are connected by heating pipelines to each other and at least some of the second heating elements are connected by heating pipelines to each other. In some embodiments, the first and second preheater units each have at least two subsets of heating elements, wherein the heating elements within each of the subset are connected by heating pipelines and the subsets are not connected by heating pipelines to each other. In some embodiments, the first and second preheater units each have three to six subsets of heating elements.

In some embodiments, the first heating elements have at least two subsets of first heating elements, wherein the first heating elements within each subset are connected by heating pipelines, the subsets are not connected to each other by heating pipelines, different heating fluids are used for heating each subset of first heating elements, the second heating elements have at least two subsets of second heating elements, the second heating elements within each subset are connected by heating pipelines, the subsets are not connected to each other by heating pipelines, different heating fluids are used for heating each subset of second heating elements, and the different heating fluids differ in a property selected from the group consisting of pressure, composition, source of the heating fluid, and temperature within the subset of first heating elements and within the subset of second heating elements. In some embodiments, the first heating elements have three to six subsets of first heating elements. In some embodiments, the second heating elements have three to six subsets of second heating elements. In some embodiments, the subsets of the first and second heating elements make use of identical different fluids. In some embodiments, the same selection of different fluids that is used for the subsets of first heating elements is also used for the subsets of second heating elements.

As used herein, “different” heating fluid refers to at least one property of the heating fluid that is different. In some embodiments, the property is selected from the group consisting of composition, temperature, and pressure. In some embodiments, different heating fluids having the same composition differ in pressure and temperature. In some embodiments, when low pressure liquid water or steam is used for a first subset of first heating elements, warmer medium pressure liquid water or steam is used for another subset of first heating elements and even warmer high pressure steam is used for another subset of first heating elements. In some embodiments, when low pressure liquid water or steam is used for a first subset of second heating elements, warmer medium pressure liquid water or steam is used for another subset of second heating elements and even warmer high pressure steam is used for another subset of second heating elements. In some embodiments, there are three different subsets of first heating elements having three different fluids, wherein the fluids are different in temperature and pressure within the subset as well as three different subsets of second heating elements having three different fluids, wherein the fluids are different in temperature and pressure within the subset. Pressure and temperature of the heating fluids increase as the reaction fluid travels towards the reaction chamber, yielding controlled warming of the reaction fluid.

In some embodiments, the low pressure liquid water or steam has a temperature in the range from 60 to 160° C., alternatively from 80 to 120° C. In some embodiments, the medium pressure liquid water or steam has a temperature in the range from 100 to 200° C., alternatively from 120 to 180° C. In some embodiments, the high-pressure steam has a temperature in the range from 160 to 300° C., alternatively 180 to 240° C.

In some embodiments, at least two subsets of first heating elements are heated with identical fluids, wherein the first heating elements within each subset are connected by heating pipelines and the subsets are not connected to each other by heating pipelines, are used and at least two subsets of second heating elements are heated with identical fluids, wherein the second heating elements within each subset are connected by heating pipelines and the subsets are not connected to each other by heating pipelines, are used. In some embodiments, the subsets of first and second heating elements with identical fluids are used in addition to the subsets with different fluids. In some embodiments, the cooling of the heating fluid within the heating pipelines is better controlled when non-connected subsets of first heating elements with identical fluids and non-connected subsets of second heating elements with identical fluids are used, that is, the same heating fluid is introduced twice into the system. In some embodiments, subsets with different heating fluids are then used in other subsets of heating elements.

In some embodiments, the amount of heating elements arranged in or around the first and second branch line is the same. In some embodiments, the length of the first and second branch line is the same.

In some embodiments, the first and second branch lines are meandering lines each having at least three bends, alternatively from four to ten bends, alternatively from five to eight bends. In some embodiments, the first and second branch lines have straight sections that are partially or fully insulated using insulation elements, wherein the bends are not insulated with the insulation elements.n

FIG. 2 shows a schematic view of a preheater unit. In some embodiments, the preheater unit is a first or a second preheater unit. In some embodiments, the preheaters element are built similarly or identically. The preheater unit of FIG. 2 is described as a first preheater unit.

The first preheater unit of FIG. 2 has a first branch line (3). The first branch line is a meandering branch line having five bends (7, 9), wherein three bends (7) are opposing another two bends (9) such that the branch line (3) leaves in the same direction as the branch line (3) entered. FIG. 2 shows a multitude of first heating elements (5, 15, 17, 19) that are arranged around the first branch line. The heating elements (5, 15, 17, 19) are arranged into different subsets, wherein within each subset the heating elements (5, 15, 17, 19) are connected by heating pipelines (27, 29, 31, 33).

In the beginning, that is, at the point where the reaction fluid enters the preheater unit as indicated by the arrows along the first branch line, a first subset of six first heating elements (5) are connected by heating pipelines (27). A low-pressure heating fluid (11) is introduced, guided though the subset of six first heating elements (5) using heating lines (27) and leaves the preheater unit at an exit port (41).

After the first subset of six first heating elements (5), another subset of six first heating elements (19) are connected by heating pipelines (29). A medium pressure heating fluid (25) is introduced, guided though the subset of six first heating elements (19) using heating lines (29) and leaves the preheater unit at an exit port (39).

After the second subset of six first heating elements (19), a subset of three first heating elements (17) are connected by heating pipelines (31). A medium pressure heating fluid (23) is introduced, guided though the subset of three first heating elements (17) using heating lines (31) and leaves the preheater unit at an exit port (37).

After the subset of three first heating elements (17), another subset of three first heating elements (15) are connected by heating pipelines (33). A high-pressure heating fluid (21) is introduced, guided though the subset of three heating elements (15) using heating lines (33) and leaves the preheater unit at an exit port (35).

The medium pressure fluids (23, 25) for the heating elements (17) and (19) comes from the same source (13). The high-pressure fluid (21) comes from a different source. The pressure and temperature are increased slowly though the first branch line such that a controlled heating of the reaction fluid is achieved.

FIG. 3 shows schematically an arrangement of a first and a second preheater unit in connection with a tubular reactor. First and second preheater units (105, 107) are shown in accordance with FIG. 2. Although the preheater units of FIG. 2 are shown, the facility may use other preheater nnits arranged in parallel. The facility of FIG. 3 has a first preheater unit (105) arranged on a first branch line (115, 117) and a second preheater unit (107) arranged on a second branch line (119, 121), wherein the first and second preheater units (105, 107) are installed in parallel. The facility also has a reactor having a reaction chamber (111). The gas is compressed using a compressor (110) and the main line (113) is divided at a junction (103) into a first branch line (115) leading into the first preheater unit (105) and a second branch line (119) leading into the second per-heater (107). The first and second brunch lines (117, 121) exit the preheater units afterwards and are combined in a merging appliance (109) before the main line (123) leads into the reaction chamber (111) of the reactor.

FIG. 3 relates to a facility for the preparation of ethylene polymers in which two preheater units (105, 107) are arranged in parallel.

EXAMPLES

Density was determined according to DIN EN ISO 1183-1:2004, Method A (Immersion) with compression molded plaques of 2 mm thickness. The compression molded plaques were prepared with a defined thermal history: Pressed at 180° C., 20MPa for 8 min with subsequent crystallization in boiling water for 30 min.

The melt flow rate MFR was determined according to DIN EN ISO 1133:2005, condition D at a temperature of 190° C. under a load of 2.16 kg.

Haze was determined according to ASTM D 1003-00 using 50 p.m thickness blown film extruded at a melt temperature of 180° C. and a blow-up ratio of 2:1.

Gloss was determined at 60° according to ASTM D 2457-03 using 50 p.m thickness blown film extruded at a melt temperature of 180° C. and a blow-up ratio of 2:1.

The gel count was determined by preparing a cast film, analyzing the film defects by an optical scanning device and classifying and counting the film defects according to size (circle diameter). The films were prepared by an extruder (type ME20) equipped with a chill roll and winder, model CR-9, and analyzed by an optical film surface analyzer with flash camera system, model FTA100 (the components were produced by OCS Optical Control Systems GmbH, Witten, Germany). The apparatus had the following characteristics

screw diameter: 20 mm;

screw length: 25 D;

compression ratio: 3:1;

screw layout 25 D: 10 D feeding, 3 D compression, 12 D metering;

dimensions: 1360×650×1778 mm³ (L×W×H; without die);

die width (slit die): 150 mm;

resolution: 26 μm×26 μm; and was operated under the following conditions

T 1 230° C.;

T 2 230° C.;

T 3 230° C.;

T 4 (adapter) 230° C.;

T 5 (die) 230° C.;

die slit die 150 mm;

take off speed 3.0 m/min;

screw speed adjusted to film thickness 50 μm;

throughput 1.0 to 1.5 kg/h (target 1.15 kg/h);

air shower on −5 m³/h,

chill roll temperature 50° C.;

vab chill roll 4 N;

winding tensile force 4 N,

draw off strength 5 N;

camera threshold threshold 1: 75%-threshold 2: 65%.

For starting the measurement, extruder and take off unit were set to the specified conditions and started with a material having a pre-determined gel level. The film inspection software was started when the extruder showed steady conditions of temperature and melt pressure. After having operated the extruder with the starting material for at least half an hour or after the gel count having reached the pre-determined gel level, the first sample to measure was fed to the extruder. After having reached a stable gel level for 45 minutes the counting process was started until the camera had inspected an area of at least 3 m² of film. Thereafter, the next sample was fed to the extruder, and after having reached again a stable gel count for 45 minutes, the counting process for the next sample was started. The counting process was set for the samples such that the camera inspected an area of at least 3 m² of film and the number of measured defects per size-class was normalized to 1 m² of film.

The draw down thickness was determined by preparing a blown film with slowly increasing take-off speed until the film had broken. The film thickness at “break point” is reported as draw down thickness. The films were prepared by a film blowing line having the following characteristics

single screw extruder with grooved feed section: 50 mm×30D

barrier screw with mixing elements at the screw tip

spiral mandrel die: 120 mm×1 mm

extruder temperature: MFR 3-0.7: 180° C.

throughput: 35 kg/hr

film blow up ratio: 2.5

single lip cooling ring (for blow up ratios from 2:1 to 4:1)

no internal bubble cooling system

height adjustable calibrating basket with Teflon® rolls

lay flat unit equipped with rolls (CFRP guide rolls, V-shape side positioning guidance by CFRP rolls; CFRP=Carbon Fiber Reinforced Plastic)

For determining the draw down thickness, the film preparation was started with a take-off speed of 3 m/min. Without changing processing conditions such as frost line, blow up ratio, and film width, the take-off speed was increased by a rate of 2 m/min every 20 seconds until the film broke or showed formation of holes. The draw down thickness was measured at a distance of 70 cm prior to the location of the film break or hole formation. A multitude of measurements was made circumferentially in distances of 3 to 4 cm and the average of these measurements was reported as draw down thickness.

Example 1

A LDPE film grade was produced by continuous polymerization of ethylene as ethylenically unsaturated monomer in a high-pressure tubular reactor of the design shown in FIG. 1 having a length of 2140 m and a design pressure of 300 MPa. The tubular reactor was composed of reactor tubes having a length of 17 m with an outer diameter do of 137 mm and an inner diameter di of 75 mm. Propionaldehyde was added as chain transfer agent to the fresh ethylene stream entering the primary compressor (1a) in an amount of 1.4 kg pert of produced polyethylene.

The secondary compressor (1b) was operated with a throughput of 125 t reaction fluid per hour. The reaction fluid was compressed to 270 MPa which was the maximum achievable before the safety reactor elements were responding. The pressure was adjusted by varying the opening periods of the pressure control valve. The reaction fluid was heated to 160° C. in a combination of two preheater units which were arranged in parallel. Both preheater units were constructed with tubes having a length of 17 m, an outer diameter do of 137 mm and an inner diameter di of 75 mm and had a length of 289 meters. The combined heat transfer area of the preheater units was 136 m². After having passed the preheater units, the fluid streams had a pressure of 265 MPa. The average velocity of the reaction fluid in the preheater units at outlets was 7.85 m/s.

The two reaction fluid streams were combined after having passed the preheater units and were fed to the reactor inlet. The average velocity of the reaction fluid in the tubular reactor at the reactor inlet was 15.7 m/s. The peroxidic polymerization initiators were metered into the tubular reactor at four positions using isododecane as additional diluent. For removing the generated heat of polymerization, cooling water was circulated through cooling the jackets attached to the outer surface of the reactor tubes. The reaction mixture discharged from the tubular reactor (4) was passed through a post reactor cooler (7) and separated from volatiles in two steps via a first separation vessel (8) and a second separation vessel (9). The degassed liquid polymer was conveyed to an extruder and granulator (10) to form LDPE pellets.

Within a period of 24 hours of continuous production, 1035 t of LDPE were obtained having the properties summarized in Table 1. Accordingly, the conversion rate of ethylene per pass through the reactor was 34.5%.

Example 2

The polymerization of Example 1 was repeated. Instead of a combination of two preheater units, a single tubular preheater constructed with tubes having a length of 17 m, an outer diameter di, of 194 mm and an inner diameter di of 106 mm was used to heat the reaction fluid stream to the temperature of 160° C. The preheater had a length of 408 meters and a heat transfer area of 136 m². The reaction fluid was compressed to 273 MPa which was the maximum achievable before the safety reactor elements were responding. After having passed the preheater, the reaction fluid stream was fed to the reactor inlet at a pressure of 260 MPa. The average velocity of the reaction fluid in the preheater at outlet was 8.56 m/s and the average velocity of the reaction fluid in the tubular reactor at the reactor inlet was 15.9 m/s.

Within a period of 24 hours of continuous production, 1050 t of LDPE were obtained having the properties summarized in Table 1. Accordingly, the conversion rate of ethylene per pass through the reactor was 35%.

Comparative Example A

The polymerization of Example 1 was repeated. Instead of operating the two preheater units in parallel, the two preheater units were arranged in serial mode resulting in a single tubular preheater having a length of 578 meters and a heat transfer area of 136 m² to heat the reaction fluid stream to the temperature of 160° C. It is believed that the higher pressure pulsations at the discharge side of the secondary compressor compared to Example 1 resulting in a maximum discharge pressure of 266.5 MPa. It is further believed that the lower compressor discharge pressure and the serial arrangement of the preheater resulted in a pressure of the reaction gas after having passed the preheater of 256.5 MPa. The average velocity of the reaction fluid in the preheater at outlet was 16.2 m/s and the average velocity of the reaction fluid in the tubular reactor at the reactor inlet was 16.2 m/s.

Within a period of 24 hours of continuous production, 965 t of LDPE were obtained having the properties summarized in Table 1. Accordingly, the conversion rate of ethylene per pass through the reactor was 32.2%.

TABLE 1 Example Example Comparative 1 2 Example A Density [g/cm³] 0.9232 0.9234 0.9231 MFR_(2.16) [g/10 min] 0.75 0.75 0.76 Haze [%] 6.5 6.4 7.4 Gloss 97 98 84 Draw down thickness [μm] 17 16 26 Gel count   <200 μm [1/100 m²] 365 350 542 200-400 μm [1/100 m²] 75 70 119 400-800 μm [1/100 m²] 6 5 11 

1. A process for the preparation of ethylene homopolymers or copolymers in a facility having i) a high-pressure tubular reactor used in a continuous flow mode with a reaction fluid comprising ethylene being introduced into the reactor at a reactor inlet and polymer leaving the reactor at a reactor outlet and ii) a preheater having an inlet and an outlet, comprising the steps of: a) compressing the reaction fluid to an elevated pressure; b) heating at least a portion of the reaction fluid in the preheater; c) introducing the reaction fluid heated in step b) into the reactor at the reactor inlet; and d) polymerizing the reaction fluid at the elevated pressure in the presence of free-radical polymerization initiators in the reactor, thereby yielding a polymer selected from the group consisting of ethylene homopolymers and ethylene copolymers, wherein the average velocity of the reaction fluid at the outlet of the preheater is lower than the average velocity of the reaction fluid in the tubular reactor at the reactor inlet and the ratio of the average velocity of the reaction fluid in the tubular reactor at the reactor inlet to the average velocity of the reaction fluid in the preheater at the outlet of the preheater is in the range from 1.5 to
 5. 2. The process of claim 1, wherein, in step a), a piston compressor is used for compressing of the reaction fluid.
 3. The process of claim 1, wherein, in step a), the pressure of the reaction fluid after compression ranges from 180 MPa to 320 MPa, or, in step c), when introducing the heated reaction fluid into the reactor, the pressure of the reaction fluid ranges from 175 MPa to 315 MPa.
 4. The process of claim 1, wherein, in step c), when introducing the heated reaction fluid into the reactor, the temperature of the reaction fluid ranges from 140° C. to 170° C.
 5. The process of claim 1, wherein the preheater has two preheater units, wherein the first preheater unit is arranged on a first branch line and the second preheater unit is arranged on a second branch line.
 6. The process of claim 5, further comprising the steps of: a′) providing a first flow of compressed reaction fluid comprising a first portion of the reaction fluid and providing a second flow of compressed reaction fluid comprising a second portion of the reaction fluid by, in any order, compressing the reaction fluid to an elevated pressure and splitting at least a portion of a single flow of the reaction fluid into the first flow and the second flow of compressed reaction fluid; b′) conducting the first flow of the first portion of the reaction fluid through the first preheater unit and heating the first portion of the reaction fluid in the first preheater unit, and conducting the second flow of the second portion of the reaction fluid through the second preheater unit and heating the second portion of the reaction fluid in the second preheater unit; c′) combining the first and second portion of the reaction fluid by recombining the first flow and the second flow of the reaction fluid and introducing the recombined flow of the reaction fluid heated in step b′) into the reactor at the reactor inlet; and d′) polymerizing the reaction fluid at the elevated pressure in the presence of free-radical polymerization initiators in the reactor.
 7. The process of claim 6, wherein, in step a′), a single flow of the reaction fluid is compressed to an elevated pressure and thereafter at least a portion of the flow is split at a junction appliance, into the first flow comprising the first portion of the reaction fluid and into the second flow comprising the second portion of the reaction fluid.
 8. The process of claim 5, wherein the first preheater unit has first heating elements and the second preheater unit has second heating elements that are arranged at intervals and spaced apart from each other.
 9. The process of claim 5, wherein the first and second heating elements are heated using a heating fluid, at least some of the first heating elements are connected by heating pipelines to each other, and at least some of the second heating elements are connected by heating pipelines to each other.
 10. The process of claim 5, wherein the first heating elements have at least two subsets of first heating elements, the first heating elements within each subset are connected by heating pipelines, the subsets are not connected to each other by heating pipelines, different heating fluids are used for heating each subset of first heating elements, the second heating elements have at least two subsets of second heating elements, the second heating elements within each subset are connected by heating pipelines the subsets are not connected to each other by heating pipelines, different heating fluids are used for heating each subset of second heating elements, and the different heating fluids differ in a property selected from the group consisting of pressure, composition, source of the heating fluid, and temperature.
 11. The process of claim 5, wherein the amount of heating elements arranged in or around the first and second branch line is the same or the length of the first and second branch line is the same.
 12. The process of claim 5, wherein the first and second branch lines have straight sections that are partially or fully insulated using insulation elements, and the bends are not insulated with the insulation elements.
 13. The process of claim 1, wherein the polymer obtained in step d) is a low-density polyethylene (LDPE).
 14. The process of claim 1, wherein the reaction fluid further comprises propionic aldehyde or propylene.
 15. The process of claim 1, wherein the reaction fluid is a supercritical fluid in steps b) to d).
 16. A process for the preparation of ethylene homopolymers or copolymers in a facility having i) a high-pressure tubular reactor used in a continuous flow mode with a reaction fluid comprising ethylene being introduced into the reactor at a reactor inlet and polymer leaving the reactor at a reactor outlet and ii) a preheater having an inlet, an outlet, a first preheater unit is arranged on a first branch line, and a second preheater unit is arranged on a second branch line, comprising the steps of: a′) providing a first flow of compressed reaction fluid comprising a first portion of the reaction fluid and providing a second flow of compressed reaction fluid comprising a second portion of the reaction fluid by, in any order, compressing the reaction fluid to an elevated pressure and splitting at least a portion of a single flow of the reaction fluid into the first flow and the second flow of compressed reaction fluid; b′) conducting the first flow of the first portion of the reaction fluid through the first preheater unit and heating the first portion of the reaction fluid in the first preheater unit, and conducting the second flow of the second portion of the reaction fluid through the second preheater unit and heating the second portion of the reaction fluid in the second preheater unit; c′) combining the first and second portion of the reaction fluid by recombining the first flow and the second flow of the reaction fluid and introducing the recombined flow of the reaction fluid heated in step b′) into the reactor at the reactor inlet; and d′) polymerizing the reaction fluid at the elevated pressure in the presence of free-radical polymerization initiators in the reactor, thereby yielding a polymer selected from the group consisting of ethylene homopolymers and ethylene copolymers, wherein the average velocity of the reaction fluid at the outlet of the preheater is lower than the average velocity of the reaction fluid in the tubular reactor at the reactor inlet and the ratio of the average velocity of the reaction fluid in the tubular reactor at the reactor inlet to the average velocity of the reaction fluid in the preheater at the outlet of the preheater is in the range from 1.5 to
 5. 17. The process of claim 16, wherein, in step a′), a single flow of the reaction fluid is compressed to an elevated pressure and thereafter at least a portion of the flow is split at a junction appliance, into the first flow comprising the first portion of the reaction fluid and into the second flow comprising the second portion of the reaction fluid.
 18. The process of claim 16, wherein the first preheater unit has first heating elements and the second preheater unit has second heating elements, wherein the first heating elements within each subset are connected by heating pipelines, the subsets are not connected to each other by heating pipelines, different heating fluids are used for heating each subset of first heating elements, the second heating elements have at least two subsets of second heating elements, the second heating elements within each subset are connected by heating pipelines, the subsets are not connected to each other by heating pipelines, different heating fluids are used for heating each subset of second heating elements, and the different heating fluids differ in a property selected from the group consisting of pressure, composition, source of the heating fluid, and temperature.
 19. The process of claim 16, wherein the polymer obtained in step d′) is a low-density polyethylene (LDPE).
 20. The process of claim 16, wherein the reaction fluid is a supercritical fluid in steps b′) to d′). 